Spikes - revisited

Back to the old topic of those spikes that at least some of us see from time to time.
I have a new neighboring station. just a few miles away, with a small “mountain” between us.

I saw a large spike this morning:

This is a Shake-Boom. The spike only occurs on the EHZ channel, no trace of it on HDF (infrasound).

So I thought I would take a look at my neighbor. He has a 3D.
His EHZ channel:

Hmmm… he sees a big spike too, but a lot later than I did. It gets more interesting looking at the other channels:

EHN:

EHE:

Not a sign of the spike on those channels.

So it is picked up by the Z channel, but not by N or E channels.
I can only conclude that is is a vertical ground movement, or, just maybe, a very polarized (strong) EMF signal.

Very strong signals, but neither one picked up by the other station, so very local, but strong.

This leads me back to an old theory of mine which is now looking likely (at least to me) again: Both devices are (as far as I am aware) sitting on large concrete floor slabs of out buildings. I think what we are seeing is expansion/contraction due to temperature changes causing stress movements within the concrete (or maybe the buildings themselves?).

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Interesting hypothesis! Mine is in the corner of a 36’ishX80ish concrete floored workshop.

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Mine is approx 30x60 pole barn - concrete floor. Not quite in the corner, but close to the wall.

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